1. Field of the Invention:
The present invention relates generally to an Orthogonal Frequency Division Multiplexing (OFDM) system. More particularly, the present invention relates to a method and apparatus for recovering a carrier frequency offset in a receiver of an OFDM system.
2. Description of the Related Art
An OFDM transmission scheme is a type of multi-carrier modulation scheme. This technology has been attracting attention along with the development of Very Large Scale Integration (VLSI) technology since the early 1990s. The OFDM transmission scheme is characterized by parallel-transmitting data on subcarriers that maintain mutual orthogonality. A general OFDM receiver 100 may be simply realized using Fast Fourier Transform (FFT) as illustrated in FIG. 1. The OFDM scheme, compared with a single-carrier modulation scheme, may efficiently use the transmission band, so it is popularly applied to the broadband transmission scheme.
In terms of the reception characteristic, the OFDM transmission scheme illustrates the robust characteristic against a frequency selective multipath fading channel, compared with the single-carrier modulation scheme. An input signal of the receiver may be a frequency selective channel in the band occupied by a plurality of subcarriers and it may also be a frequency nonselective channel in each subcarrier band. Therefore, the input signal can be easily channel-compensated through a simple channel equalization process. In particular, the OFDM scheme generates a cyclic prefix (CP) by copying a rear part of each OFDM symbol and transmits the CP in front of the corresponding OFDM symbol, thereby removing inter-symbol interference (ISI) from a previous symbol. The OFDM transmission scheme is suitable for broadband high-speed communication as a result of its robust characteristic against multipath fading.
In the digital broadcast standard, the OFDM transmission scheme has attracted attention as a transmission technique capable of guaranteeing high reception quality and high-speed transmission/reception. Digital Audio Broadcasting (DAB) for European wireless radio broadcasting and Digital Video Broadcasting-Terrestrial (DVB-T) which is the terrestrial High Density Television (HDTV) standard are examples of the broadcast standard using the OFDM transmission scheme. Recently, the development of mobile broadcast systems is under way all over the world along with the mainstream of communication-broadcasting convergence. The main object is to transmit high-capacity multimedia information in the mobile channel environment. While Europe has established Digital Video Broadcasting-Handheld (DVB-H) developed from DVB-T as the European mobile broadcast standard, South Korea has established Terrestrial Digital Multimedia Broadcasting (DMB) developed from DAB as the broadcast standard. Even the European DVB-H is officially recognized as another standard of the domestic Terrestrial DMB, and MediaFLO proposed by the Qualcomm, USA and the Japanese ISDB-T system are also based on the OFDM transmission scheme.
A synchronization algorithm of the OFDM system is roughly classified into a carrier frequency synchronization algorithm and a symbol-timing synchronization algorithm. The carrier frequency synchronization algorithm corrects a carrier frequency offset between a transmitter and a receiver. The carrier frequency offset generally occurs because of a difference in oscillator frequency between the transmitter and the receiver, and a Doppler effect due to movement of the receiver. A Carrier Frequency Offset (CFO) of a signal input to the receiver can be greater than subcarrier spacing. A process of correcting an Integer Frequency Offset (IFO) corresponding to an integer multiple of the subcarrier spacing is defined as initial carrier frequency synchronization, and a process of correcting a Fractional Frequency Offset (FFO) corresponding to a fractional multiple of the subcarrier spacing is defined as fine carrier frequency synchronization. The fine carrier frequency synchronization is also defined as a step of correcting the residual frequency offset after the initial carrier frequency synchronization.
Because the offset IFO corresponding to an integer multiple of the carrier shifts a transmitted OFDM signal by an integer multiple of a subcarrier in a frequency domain, it serves to shift an FFT output sequence by the integer multiple. Alternatively, the offset FFO corresponding to a fractional multiple of the subcarrier causes interference between FFT outputs, thereby causing considerable bit error rate (BER) performance degradation. The OFDM system, compared with the single-carrier transmission system, is higher in the performance degradation due to the carrier frequency offset.
The conventional initial carrier frequency synchronization algorithm that estimates IFO can be divided into a blind detection scheme and a scheme using predefined (or previously engaged) symbols. As an example of the blind detection scheme, there is an algorithm that uses a scheme of estimating a shift of a signal band using a guard band. However, because this algorithm illustrates considerable performance degradation in the multipath fading channel environment, it is hard to actually implement the algorithm. Alternatively, the scheme using the predefined symbols transmits predefined symbols separately from data symbols, thereby causing a decrease in data rate. However, because this scheme improves synchronization and channel estimation performance, it is substantially applied to most OFDM systems.
In the OFDM system, an initial carrier frequency recovery apparatus using predefined symbols in the frequency domain can generally be divided into a structure 200 for correcting a frequency offset in a digital domain as illustrated in FIG. 2A, and a structure 220 for correcting a frequency offset in an analog domain as illustrated in FIG. 2B. OFDM signals received via an antenna are converted into baseband signals by radio frequency (RF) receivers 201 and 221, and then converted into digital-domain signals by Analog-to-Digital Converters (ADC) 203 and 223. Thereafter, the digital-domain signals are converted into frequency-domain signals by Fast Fourier Transformers (FFTs) 207 and 225. Frequency offset estimators 209 and 227 estimate frequency offset values using characteristics of predefined symbols and then correct the estimated frequency offset values.
The predefined symbols transmitted for synchronization and channel estimation of the receiver are generally composed of a pseudo-random nose (PN) sequence and a sequence capable of using an auto-correlation characteristic. According to the auto-correlation characteristic, an auto-correlation value illustrates the maximum value when there is no code offset between a sequence transmitted at an input predefined symbol and a sequence locally generated in the receiver, and the auto-correlation value decreases when there is the code offset.
FIG. 3 is a diagram illustrating frequency estimation performance in the conventional frequency offset estimation process. Specifically, FIG. 3 illustrates IFO detection error probability with respect to Signal-to-Noise Ratio (SNR). Herein, the horizontal axis represents SNR, and the vertical axis represents IFO detection error probability. In addition, ΔFc denotes a frequency offset. In the following description, IFO and FFO will be used as a carrier frequency offset corresponding to an integer multiple and a carrier frequency offset corresponding to a fractional multiple, respectively, both normalized at subcarrier spacing.
As illustrated in FIG. 3, as FFO approaches 0.5, IFO detection error probability increases abruptly, and if FFO is 0.5, the IFO detection error probability converges at ½ as illustrated by a circle 301. Therefore, even though the fine carrier frequency recovery process is performed, the finally estimated frequency offset value may have a difference corresponding to the subcarrier spacing.
In addition, because the fine carrier frequency recovery process generally uses a phase detector whose pull-in range is −0.5˜0.5 (unit: subcarrier spacing), if FFO is in the vicinity of 0.5, a hang-up phenomenon occurs. The hang-up phenomenon causes an increase in the convergence time. If a sine-type phase detector's characteristic curve is used, an increase in the convergence time due to the hang-up phenomenon is more considerable.
Accordingly, there is a need for an improved apparatus and method for improving carrier frequency estimation performance in an OFDM system.